<PAGE>
 
                                          Exhibit (10)-20
                                          Unicom Corporation and
                                          Commonwealth Edison Company
                                          Form 10-K File Nos. 1-11375 and 1-1839



                              SECOND AMENDMENT TO
                             EMPLOYMENT AGREEMENT


  This SECOND AMENDMENT (the "Second Amendment") dated as of January 27, 1999 to
the Employment Agreement dated as of March 10, 1998 by and among the parties
hereto (the "Employment Agreement"), as subsequently amended by the First
Amendment to Employment Agreement (as so amended, the "Existing Employment
Agreement"), is made by and among Unicom Corporation, an Illinois corporation
("Unicom"), Commonwealth Edison Company, an Illinois corporation ("ComEd"), and
John W. Rowe ("Executive").

  WHEREAS, the Compensation Committee of the Board of Directors of each of
Unicom and ComEd and Executive have determined that it is desirable to amend
that portion of the Existing Employment Agreement relating to Executive's Annual
Incentive; and

  WHEREAS, such Compensation Committees approved and adopted such amendment at
their meetings on January 27, 1999;

  NOW THEREFORE, in consideration of their mutual undertakings set forth in this
Amendment, the parties hereto agree as follows:


                                       I

  Unless otherwise specifically defined herein, each term used herein which is
defined in the Existing Employment Agreement shall have the meaning assigned to
such term in the Existing Employment Agreement.  Each reference to "this
Agreement" and each other similar reference contained in the Existing Employment
Agreement shall refer, from and after the date hereof, to the Existing
Employment Agreement as amended hereby.


                                       II
<PAGE>
 
  The last sentence of Section 4.3 is hereby amended to read as follows:

          "During each of 1998 and 1999, the Annual Incentive shall be paid in
          cash as to the first $300,000 thereof and the excess, if any, shall be
          paid 50% in cash and 50% in shares of Common Stock."


                                      III

  Except as amended herein, the Existing Employment Agreement remains in full
force and effect.

                                       2
<PAGE>
 
                                       IV

  This Second Amendment may be executed in any number of counterparts, each of
which shall be deemed to be an original, but all of which together shall be
deemed to be one and the same instrument.

  IN WITNESS WHEREOF, the parties have executed this Second Amendment as of the
date first written above.


                                            UNICOM CORPORATION
 
                                  By:______________________________________
                                               Edward A. Brennan,
                                          Chairman of the Compensation
                                        Committee of the Board of Directors
 
                                  By:_______________________________________
                                                Donald P. Jacobs
                                           Lead Non-employee Director
 
 
                                  COMMONWEALTH EDISON COMPANY
 
                                  By:_______________________________________
                                               Edward A. Brennan,
                                          Chairman of the Compensation
                                        Committee of the Board of Directors
 
                                  By:_______________________________________
                                               Donald P. Jacobs
                                         Lead Non-employee Director
 
 
                                  EXECUTIVE:
 
                                  __________________________________________
                                          John W. Rowe

                                       3